This invention relates to concentric cylinder viscometers which are also plastometers, as used for determining the rheological characteristics of plastic and in particular thixotripic fluid, such as are used for example in petroleum well drilling and production, such as rotary drilling fluids, packer fluids, gravel and placement fluids, and hydraulic fracturing fluids.
Newtonian fluids such as most pure liquids of relatively low molecular weight undergo a finite shear when a finite shearing stress is applied, no matter how small the latter may be. For plastic fluids in general, there exists a shearing stress at and below which no shear takes place. Other fluids of intermediate character may have this limiting shear stress approach zero, but nevertheless exhibit a nonlinear relation between applied shearing stress and resulting shear. Some fluids which are not Newtonian may also exhibit a minimum shearing stress of the type described, that is, below which no shear takes place so that the fluid behaves essentially as a solid; and still other fluids may be of such a character that the minimum shearing stress increases with the time of quiescence.
It will be clear from the foregoing that fluids of the type described may be characterized by a number of parameters, and that these may have a considerable range of values. In the practical utilization of these fluids, accordingly, it is essential to be able to characterize a given fluid with respect to all of these parameters, and to do so in a reproducible and reliable fashion.
Some of the devices which have been used in various fields for the rheological characterization of non-Newtonian fluids are described in the text "Viscosity and Flow Measurement" by J. R. Van Wazer et al, Interscience Publishers, New York, 1963. Pages 156-161 of this text, hereby included herein by reference, relate to a type of concentric cylinder rheometer which has been widely used in applied petroleum technology, and which is described additionally in U.S. Pat. No. 2,703,006, to Savins.